Pulse radar systems transmit impulses of RF energy and measure the time for pulse echoes from the target to return, from which the range to the target is estimated.
Continuous wave (CW) radar systems continuously transmit RF energy and receive back reflection from a target, and provide velocity as well as range information by comparing differences in the transmitted and received waves and using the Doppler effect.
In frequency modulated CW (FMCW) radar systems the continuous wave RF energy is frequency modulated, and the signal reflected from a target mixes with the transmit signal to produce a beat signal from which range may be determined. Usually the beat signal is digitally sampled and then passed through a discrete fourier transform (DFT) in order to determine the frequency spectrum of the beat signal.
Advantages of FMCW radar over pulse radar include low power consumption and emission, high close range sensitivity and resolution, and fast start up time. The low power emission of FMCW radar facilitates its use in small marine vessels and boats. In 2008 the Navico group introduced to the market a range of FMCW radar units for the recreational marine market. An advantage of FMCW radar over CW radar is that CW radar cannot determine range to a stationary target (zero Doppler effect).
The range discrimination of a FMCW radar system is inversely proportional to the bandwidth of the frequency modulated waveform. In practical systems a window function must be applied to the beat signal in order to reduce the effect of frequency (range) side-lobes. This reduction of range side-lobes comes at the expense of a degradation in range discrimination. A window function that provides a high degree of side-lobe reduction will also cause a high-degree of degradation of the range discrimination. Therefore, in practical systems the best case range resolution is not achieved.
FMCW radar systems are also subject to interference from other radars. These radars may include other FMCW radars, pulse radars or pulse compression radars which are operating nearby. A simple technique for removing interference is to blank (e.g. set to zero) the samples of the beat signal where interference is present. However this technique often introduces undesirable artifacts into the signal which can be worse than the original interference.